"We also believe that our study will become a useful reference for future theoretical and experimental studies of asymmetric plasmonic nanostructures," team leader Roy Johnston told nanotechweb.org.

Metallic nanostructures are a key component in many nanoscale devices, such as electrodes and magnetic systems, including recording media and photonic devices. The optical properties of these materials are directly related to so-called localized surface plasmon resonances (LSPR), which depend on collective excitations of conduction electrons at the metallic surface.

The electromagnetic field surrounding a metallic nanoparticle that has been excited by light at frequencies near the plasmon resonance frequency can extend over quite large distances. This phenomenon can thus be compared to the wavefunctions of simple atoms, say scientists: just like atoms in molecules and solids, metal nanoparticles can assemble to create a diverse range of new plasmonic nanoclusters, such as dimers, trimers, tetramers and so on. And each type of cluster has its own unique set of plasmon modes – that is, collective oscillations of electrons produced by the light.

Dipole-dipole interactions
Researchers usually describe such modes using dipole-dipole interactions whereby two nearby oscillators couple together. In the case of two adjacent metallic spheres, a lower-energy resonance corresponds to two dipoles aligned parallel to each other. This leads to a strong red-shifted absorption peak in the optical spectrum of these materials. For higher-energy resonances, the coupled dipoles cancel each other out, which results in a resonance with a zero net dipole moment that does not interact with incoming light and so does not appear in the optical absorption spectrum of the particle pairs.

Such descriptions have allowed us to better understand the electromagnetic properties of plasmonic materials but a complete explanation is still lacking, says Johnston. This is why his group decided to study asymmetric particle dimers and include effects not present in symmetrical configurations.

The team employed the "finite difference time domain" (FDTD) method to compute how plasmonic nanostructures made of silver, gold and copper nanosphere heterodimers responded to light shone on them. The dimer systems studied are constructed of spherical nanoparticles with a diameter of 60 nm and each particle is spaced between 3 to 15 nm from its neighbour. The researchers looked at how incident light polarization angle and interparticle separation affected the modes observed in the scattering spectra of the dimers. They then determined the charge distributions on the surface of the structures from calculated electromagnetic fields.

Antiphase plasmon modes
Johnston and colleagues confirmed that longitudinal coupling between the nanoparticles depends on interparticle spacing, as observed in earlier studies, but they also found something new. "We observed novel 'antiphase' collective plasmon modes at lower energies in the scattering spectra of the heterodimers," explained Johnston. "This result has potential implications for developing optical devices operating in the visible region of the electromagnetic spectrum, such as negative permittivity optical nanodevices."

The team now plans to compute the effect of symmetry breaking on these heterodimers – by changing the shapes of the particles or their compositions.

The current results are reported in AIP Advances.